The last time you saw a bat, it probably flew by in an instant, a whizzing grey ball of fur in the night.
A group of biologists led by Nicolai Konow of Brown University recently decided to take a much closer look at how exactly bats manage to be the only mammals truly capable of sustained flight.
To do so, they took ultrahigh-speed X-ray videos of fruit bats as the creatures lifted themselves off the ground and discovered that their extra-stretchy bicep and triceps tendons are crucial for storing and releasing the energy needed for takeoff. They made the videos, in all their slow-motion glory, freely available for your enjoyment.

The team specifically looked at a species called Seba’s short-tailed bat, and used technology called XROMM (X-ray Reconstruction of Moving Morphology) that integrates three-dimensional renderings of bone structure into X-ray video, allowing for detailed analysis of muscle mechanics and anatomy during an animal’s movement. Their analysis—which they presented today at a meeting of the Society for Experimental Biology—showed that the bats first stretch out the tendons that anchor their biceps and triceps muscles to their bones, then compress the tendons to release energy and power their flight upward.
This finding was confirmed by another innovative method of studying the fruit bat’s anatomy in motion: a technology called fluoromicrometry, in which chemically labeled markers are injected into the animal’s muscles. These let the researchers directly measure changes in the length of the muscles during contraction and expansion as part of flight. Calculations showed that energy output associated with the changes in muscle length alone couldn’t provide enough power for flight—further pointing towards the role of stretchy tendons.
The discovery comes as something of a surprise to biologists, who previously believed that small mammals have tendons that are too stiff and thick to be stretched at all. But this capability—and their associated ability to fly—provides further evidence that bats are truly unique among their kind.